NMR study of the impurity induced ordered state in the doped Haldane chain compoundSrNi1.93Mg0.07V2O8

Abstract

We report the effect of Mg $(S=0)$ substitution at the Ni site of $S=1$ Haldane chain compound ${\text{SrNi}}_{2}{\text{V}}_{2}{\text{O}}_{8}$ from magnetic susceptibility and $^{51}\text{V}$ NMR studies. The magnetic-susceptibility results in presence of different external fields ($H=0.1$, 1.198, 3, and 7 T) in ${\text{SrNi}}_{1.93}{\text{Mg}}_{0.07}{\text{V}}_{2}{\text{O}}_{8}$ indicate a strongly field dependent nature of the antiferromagnetic (AF) ordering in this compound, with the ordering being suppressed at $H\ensuremath{\ge}3\text{ }\text{T}$. A comparison of these results with those reported in isostructural compound ${\text{PbNi}}_{1.76}{\text{Mg}}_{0.24}{\text{V}}_{2}{\text{O}}_{8}$ indicates a stronger interchain exchange in the Sr compound. In order to probe the local magnetic properties of ${\text{SrNi}}_{1.93}{\text{Mg}}_{0.07}{\text{V}}_{2}{\text{O}}_{8}$ both in the AF ordered state and also when the ordering is suppressed, the $^{51}\text{V}$ NMR studies were performed in presence of the magnetic fields of $H=1.198$ and 7.04 T. The resonance line shape in presence of both the fields remains almost unaltered in the temperature range of 50--300 K, as was reported in the pure system. However, the linewidth increased appreciably in the range $4\ensuremath{\le}T\ensuremath{\le}20\text{ }\text{K}$ in presence of both the fields. The temperature dependence of the first moment $({M}_{1})$ and the second moment $({M}_{2})$ of the NMR spectra indicates the effect of the development of three-dimensional (3D) correlations among the staggered spins below 20 K when $H=1.198\text{ }\text{T}$. Whereas, the behavior at $H=7.04\text{ }\text{T}$ indicates the more dominance of the short-range correlations among the staggered spins within the chain in the same temperature range. The behavior of the spin-lattice relaxation rate and the dynamic susceptibility in the same temperature range further support these findings. Moreover it is seen that the intrachain exchange $(J=106\text{ }\text{K})$ and the spin gap $(\ensuremath{\Delta}=25\text{ }\text{K})$ for the uninterrupted chain remain almost unchanged after Mg substitution. Finally the present results suggest the coexistence of spin gapped phase of the uninterrupted chain together with the 3D correlated impurity induced staggered spins at $H=1.198\text{ }\text{T}$ in ${\text{SrNi}}_{1.93}{\text{Mg}}_{0.07}{\text{V}}_{2}{\text{O}}_{8}$.